A crystallographic and mössbauer spectroscopy study of Fe2+3Al2Si3O12-Fe2+3Fe3+2Si3O12, (almandine-“skiagite”) and Ca3 Fe3+2Si3O12-Fe2+3Fe3+2Si3O12 (andradite-“skiagite”) garnet solid solutions

Abstract

The crystal chemistry of garnet solid solutions on the Fe 3 2+ Al2Si3O12-Fe 3 2+ Fe 2 3+ Si3O12 (almandine-“skiagite”) and Ca3Fe 2 3+ Si3O12-Fe 3 2+ Fe 2 3+ Si3O12 (andradite-“skiagite”) joins have been investigated by single-crystal X-ray structure refinements and Mossbauer spectroscopy. Together, these two solid solution series encompass the complete range in Fe3+/ΣFe from 0.0 to 1.0. All garnets are isotropic and were re0fined in the Ia % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqaqFfpeea0xe9Lq-Jc9% qqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabG4mayaara% aaaa!3622! $$\bar 3$$ d space group. Small excess volumes of mixing are observed in andradite-“skiagite” solid solutions (W v =1.0±0.2 cm3 mol-1) and along the almandine-“skiagite” join (W v =-0.77±0.17cm3 mol-1). The octahedral (Al, Fe3+)-O bond lengths show a much greater variation across the almandine-skiagite join compared to the andradite-skiagite garnets. The dodecahedral (X)-O bond lengths show the opposite behaviour. In andradite-“skiagite” solid solutions, the octahedral site passes from being flattened to elongated parallel to the 3 axis of symmetry with increasing “skiagite” content. A perfect octahedron occurs in a composition of ≈35 mol% “skiagite”. The occupancy of the neighboring dodecahedral sites has the greatest effect on octahedral distortion and vice versa. The Mossbauer hyperfine parameters of Fe2+remain constant in both solid solutions. The hyperfine parameters of Fe3+ (at room temperature: centre shift=0.32–0.40 mm/sec, quadrupole splitting (QS)≈0.21–0.55 mm/ sec) indicate that all Fe3+ is in octahedral coordination. The Fe3+ parameters are nearly constant in almandine-“skiagite” solid solutions, but vary significantly across the andradite-“skiagite” join. The structural unit that contributes to the electric field gradient of the octahedral site is different from that of the coordinating oxygen polyhedron, probably involving the neighboring dodeca-hedral sites. Fe3+/ΣFe area ratios derived from the Mossbauer spectra systematically overestimate Fe3+ contents in both solid solutions series. This is attributable to different recoil-free fractions for Fe on the octahedral and dodecahedral sites. A correction has been derived that yields more accurate Fe3+/ΣFe ratios from room temperature and 80 K Mossbauer spectra.